1. Field of the Invention
The present invention relates to an image forming apparatus capable of obtaining a toner image excellent in glossiness.
2. Description of the Related Art
Up to now, widely known is an image forming apparatus employing an electrophotographic process such as a copying machine, a printer, a facsimile machine, and a combined machine having functions thereof. In addition, a large number of image forming apparatuses for performing not only a monochromatic image formation but also a full-color image formation are commercially available.
Further, with an increase in use of the image forming apparatuses employing the electrophotographic process in various fields, demands for higher image quality have increased. Herein, one of the factors that determines an image quality, particularly, glossiness of a full-color image is smoothness of an output image.
To meet such the demands, there has been proposed an image forming method for forming a color image in which color toner composed of a thermoplastic resin is transferred onto a recording material (i.e., glossy dedicated paper) having a transparent resin layer composed of the thermoplastic resin, and is heated and melted. As a fixing device preferably used in carrying out the image forming method, there is a belt fixing device. The belt fixing device has a structure in which a recording material bearing a toner image is pressed and heated by a fixing belt formed of a heat-proof belt, the toner image is cooled to be solidified in a state where the recording material is brought into contact with the fixing belt, and the recording material having the toner image fixed thereon is detached from the fixing belt. When such the belt fixing device is used, the toner image is embedded in the transparent resin layer of the recording material. Thus, the entire surface of the recording material becomes smooth, so it is possible to obtain a color image excellent in glossiness.
Such the image forming method has a problem with how to obtain a surface of the recording material having toner formed thereon with smoothness and without a boundary between the toner and the surface of the recording material. JP 2002-091212 A describes that a defect in smoothness at a boundary between images is improved by optimizing a hardness of a belt, thereby making it possible to obtain a smoother recording surface. In addition, JP 2004-205563 A describes that a blistering of a resin layer of the recording sheet is prevented and a rugged appearance (i.e., bump α shown in FIG. 4) of a toner image formed on a recording sheet is eliminated by regulating a fixing temperature of the belt fixing device within a certain constant range according to a softening temperature of the resin layer of the recording sheet.
As described above, even when the glossy dedicated paper having the resin layer provided thereon and the belt fixing device are used, it is difficult to completely eliminate the defect in glossiness due to the bump α of the toner. In particular, it is difficult to eliminate the defect in glossiness due to a dimple S1 (shown in FIG. 4D) which is generated by an effect of the bump of the toner image obtained before the toner image is fixed by the belt fixing device.
To be specific, in FIG. 5C is a schematic cross-sectional view showing a state of a recording material bearing a toner image immediately before entering a fixing nip portion N2 of the belt fixing device. A space denoted by symbol S is blocked by the recording material and the belt on four sides thereof. In other words, a lower portion of the space S is blocked by a resin layer P2a of a recording material P, an upper portion of the space S is blocked by a fixing belt 64, an upstream of the space S in a moving direction of the recording material is blocked by a toner layer T, and a downstream of the space S in the moving direction of the recording material is blocked by a fixing nip portion N2, respectively. Thus, the resin layer P2a cannot be brought into close contact with a surface of the fixing belt 64. With the presence of the space S, on the surface of the recording material after being fixed with the toner image by the belt fixing device, the dimple S1 shown in FIG. 5D is generated.
To suppress generation of such the dimple S1, it is sufficient to narrow the space S. As described above, in order to narrow the space S, improvements such as selection of the belt with an optimum hardness, and optimization of a heat deformation quantity of the resin layer of a recording sheet have been promoted. However, it is difficult to suppress the effect of the space S.